Process for removing dissolved silica from water



Fetented ct l3}, ice

- sits 222s:?@? lPR GCESS FOE RJEMGVENG DESSEQDENEE? 86A. FltttflliiWA'lllEhi Otto Liebhnecht, Eotsdam-Efiehelshesg, Germany, asslgnor toThe leutit Company, New Sloth, N. 2, a corporation ct Deleware No Dre.

Application June ii, 3.9%,

No, 339,$91. Kn linens; id, 3% r (Ci. Zit hit) 13 Claims.

This invention relates to a process for removdissolved, silica fromwater; and it comprises bringing the silica containing water in contactwith granular burned mcgnesite, regenerating the magnesite with a diluteacid after exhaustion of its silica removal capacity, and using theacidtreated burned magnesite again for the removal In the art of waterconditioning, especially in the treatment of water for use as boilerfeed water and similar purposes, the desirability of low crime thesilica content has been recognized for a long time, It is well knownthat silica. in the feed water leads to the formation of boiler scalewhich causes poor heat transmission. To avoid the formation of suchscale it is either necessary to maintain so high. an alkalinity in theboiler water that silica precipitation cannot take place, or to removethe silica from the feed water completely or at least reduce it to apractically negligible quantity. Prevention of silica scale bymaintaining a high alkalinity has the disadvantage of promoting causticembrittlement with consequent weakening of the boiler metal, and thesilica is readily carried over in the steam and then is responsible forthe formation of troublesome deposits on the blades of steam turbines.In using water for purposes other than boiler feed, such as textileprocesses generally, and dyeing specifically, the silica contentlikewise causes difilculties.

Various methods have been suggested for removing silica from water. Onesuch method comprises contacting the water with insoluble gels of oxidesof metals, such as aluminum and iron, and removing the silica from thesegels by regeneration in order to make them suitable for further use.Another suggestion along these lines contemplates the formation of gelsor gel mixtures in situ by adding to the water substances required forthe formation of such gels, viz.' a

metal salt end an elite-ii, such as ammonia, or h-v feeding chemicals,such as the insoluble oxides and hydroxides of magnesium, zinc, cadmium,copper, manganese or the like, which upon col,- teot with the water formthe desired gelsL-fi None of these suggestions has been whollysatisfactory in erect-loci Either the silics content is not reduced to csuiiicientiy lot: value, or

the gels cannot he regenerated completely, it being impossible to removeall silica from the gels so their capacity for efliclent silica removalis impaired,

it have discovered that he used in a cychc process for elite Suchresigns-site hes the property silica from water with which it is broughtin cotntect, gorei'erobly by allowing the water to per colcte through aReed of this materiel in granular form; When the capacity of the hurriesmaglie-site to take up silica, has been exhausted it may he subjected toe regeneration treatment withv an acid which restores its capacity torabsorption silica, and thus renders the magnesite suitshic for furtheruse in removing: silica from water.

The term burned magnesite used to denote a sintered megnesite which hasbeen heated to a temperature sufficiently high (about 3200 F.) tosubstantially reduce its solubilit in dilute strong acids, such ashydrochloric acid; it is also used to denote a magnesite which has beenburned at lower temperatures, as those used in the preparation ofcaustic magnesite (upwards of 1156" F.) Magnesite burned at such lowertemperatures is attacked by water containing cerbonate and therebyextensively transformed to magnesium carbonate; it should, therefore,only be used in the silica removal treatment of water free or freed ofcarbonates; and in regeneration, milder acids, such as acetic acid,should he em ployed.

I am aware of the use of marble. dolomite, magnesia, caustic megnesiteand sintered magmesite suggested for deacidiflcatlon of water,especially removal of carbon dioxide. In a water tie-acidificationprocess the free C02 in the water reacts with the dolomite or magnesiteto dissolve ltas bicarbonate or carbonate. In addition, substentlalquantities ofmagnesiuln compound are dissolved in the form of hydroxidemaking the for complete water more or less alkaline, according to themagnesium hydroxide concentration. Such processes of de-acidification,however, difier fundamentally from silica removal in accordance with myinvention in that the treatment medium may be used indefinitely withoutrequiring regeneration, and is, in fact, not regenerated. Any incidentalreduction of the silica content of the water being de-acidified whichmay take place while the acid removal medium is fresh, soon comes to anend.

For silica removal by means of burned magnesite in accordance with myinvention it is advantageous to preheat the water since at elevatedtemperatures silica removal takes place more rapidly and, as a rule,more thoroughly.

The grain size of burned magnesite best suited for silica removal variessomewhat. In general, grains of about 0.2 to-0.5 mm. diameter have ivengood results.

Regeneration of the magnesite is carried out when its silica removalcapacity has been ex- ,hausted to such extent, that the silica contentofv the treated water has reached the maximum limit considered allowablefor the purpose for which the water is to be used. The regeneration isbest carried out with dilute acid in order to avoid attack on themagnesite grains proper. The use of dilute hydrochloric acid has beenfound to give good results in regeneration, but other mineral acids,such as nitric, sulfuric, or carbonic acid, as well as organic acids,such as I acetic or lactic acid are likewise suitable.

One good way of carrying out the regeneration is to pass the acidupwardly through the bed of exhausted burned magnesite, since thisfacilitates the escape of gases, such as CO2, which, as a rule, areliberated during regeneration. If desired, the material may be removedfrom the container in which the silica removal treatment takes place,for regeneration elsewhere.

The amount of acid required for regeneration is usually quite small.When the acid is passed through exhausted magnesite the regeneratingreactions neutralize eiiluent is alkaline. In order to restore to theburned magnesite its full silica removal capacity it is, of course,necessary to regenerate it com- P tely, and an indication that this hasbeen accomplished is available in the appearance of acid in theefiiuent. The exact quantity of acid to be used for regeneration is thusreadily determined in each case by continuing the flow of acid untiltraces of waste efliuent.- As

acid begin to appear in the stated above, during regeneration gases areusually liberated, and it is clear, therefore, that the regenerationprocess has not been carried to completion as long as gases escape fromthe magnesite undergoing regeneration. Thus, another criterion fordetermining the minimum quantity of acid required regeneration is foundin. the fact that there should be no liberation of gases from the bed atthe end of the acid treatment.

The acid is best used in quite dilute form, a concentration of to 2 percent giving good results. The rate at which the acid is passed throughthe magnesite should be such that the total time of contact between acidand magnesite is of the order of about minutes to one hour.

Prior to regeneration it is advisable to loosen e magnesite bed by meanswell known to accomplish this, such as backwashing, surface washing,

the acid completely and thefor this purpose.

aecegrcv rakes moved through the bed. When the regeneration is carriedout by flowing dilute acid upwardly through the bed such looseningoperation prior to regeneration may be dispensed with, especially if thewater being treated is relatively clear, because the upflowing acid thenperforms a backwashing function.- In cases where a water with highturbidity, especially one with relatively low silica content, is treatedfor silica removal it may be necessary to carry out one or even severalbed loosening operations between normal regenerations since the silicaremoval capacity or burned magnesite is high, and the treatment runs,therefore, relatively long.

After completion of the acid treatment the magnesite is freed of anyremaining free acid by rinsing it with water, the efliuent beingdischarged to waste until it is entirely free of acid and shows asufiiciently low silica content, where'- upon the magnesite may again beused for further silica removal.

In using burned magnesite for silica removal with subsequentregeneration some magnesite is consumed. In order to make up for thisloss it is necessary to replenish the burned magnesite at infrequentintervals. As far as I have been able to ascertain, the magnesiteconsumption appears to be approximately equal to the magnesium bound tosilica in the form of magnesium silicate, plus an amount consumed inneutralizing CO2, as well as other side reactions, evidenced by areduction of the alkalinity, if any, of the water being treated forsilica removal. It follows, therefore, that in cases where a water withappreciable alkalinity is to be treated the magnesite loss can bereduced by pre-treating the Water so as to reduce or completely removeits alkalinity by any one of the methods well known In this connection,good ways of reducing the alkalinity are softening by precipitation, orby a combination of hydrogen and sodium base exchange, or neutralizingalkalinity by the addition of acid or acid salt, such as sulfuric acidor sodium bisulfate.

My process may be combined with other silica removal processes. Thus,water may be partially freed of its silica by other methods, as forinstance the gel methods referred to above, and then passed through abed of burned magnesite in order to substantially remove all remainingsilica. Or, the water may be pre-treated by feeding to it silicaprecipitating or binding agents, as for instance the hydyroxides ofmetals such as iron and aluminum, or by forming such agents in the waterprior to its, passage through a bed of burned magnesite; the magnesitebed, in addition to removing the remaining silica as described, thenacts as a filter and collects the precipitated matter which isperiodically removed from the bed by backwashing.

Other examples of pre-treating the Water before silica removal by meansof burned magnesite or by mechanical means as, for instance,

are the addition of caustic magnesium compounds, such as MgO or Mg(OH)2,in the form of milk of magnesia; or else the water may first be passedthrough a magnesite filter which mainly serves the purpose ofneutralizing the water. The thoroughness of removing silica from waterby my new process, and the relatively small amount of acid required forregeneration, of the magnesite are illustrated inthe following examp e:i

10.6 cubic feet of burned grain size of 0.2 to 0.5 in a bed 50 inchesdeep magnesite having 9. mm. diameter was used to remove silica from ato *1'5 partsper million anda total hardness of "17910 215 parts permillion, of which two-thirds was carbonate. and one-third non-carbonatehardness.

The water was preheated to a temperature of 176 to 189 ,F. and thenpassed through the bed of the burned magnesite at a flow: rate of' about'1060 to 1320 gallons per v hour: The silica was removed so thoroughlythat *even. at the 'end of the test, after treating about burnedmagnesite had first been regenerated with 240 gallons of hydrochloricacid solution made from commercial hydrochloric acid and contain--";ing3'7'.4 pounds of HCl. The progress of the run is shown in thefollowing table:

m t of dlkalinigtgf treated water n i Gallons of water treated n p perIon treated water in h 1' i pa 5 per P 'eno Meth 1 million .phthalein muTotal 0. s 2. 5 s. o 10. 0. 5 2. 5 8. 5 11. o. s 2. 5 8.0 10. 0. 5 2. 38. 2 10. 0. 7 2. 3 8. 2 10. 0. 7 2. 5 8. 2 10. 0. s 2. 5 8.0 10. 0. 8 2.5 8. 2 i 10.

$ince the untreated water had a total alkalinity of lfito 16 parts permillion the test results 7 show that an alkalinityreduction of aboutonethird of 'the carbonate content took place hand in hand' witli t hesilica removal. The hardness of the treated-water was about 144 parts.per million practically all magnesium hard- FF r magnesite which .wasused in this test had tliiollowing composition:

cates are present ina colloidal state. 24,000 gallons, the silicacontent of the treated water did not exceed one part per million. The

results in the previously described hot water test. In making silicaremoval tests at ordinary temperatures I found that directly afterregeneration of the burned magnesite the treated water has a high S102content, occasionally even higher than that of the untreated water; onlyafter some time does the silica content of the treated water drop to thedesired low value. This appears to be due to the fact that afterregeneration the sill- While this presence of larger amounts of silicain the treated water directly after regeneration is less pronounced whenthe silica content of the burned magnesite is low (1 to 2 per cent) thanwhen it 1 is'higher (5.5%) so that the use of .magnesite containing noor but little silica is desirable for that reason, this difficulty of ahigher silica content in .the treated water at the beginning of a runmay be overcome by coagulation of the silica 0 or silicates. Suchcoagulation treatment associ-' ated with the process of regenerating theburned magnesite may take one of several forms.

One way of overcoming this difdculty is to allow the burned magnesite tostand at rest for some time after regeneration. I found that a bed ofburned magnesite regenerated in the evening and standing idle overnightgave on the following morning an eiiluent with a satisfactorily lowsilica content. This suggests a method of operation including a periodof idleness following '1 egeneration of the magnesite.

be provided because when a plant.does not require a continuous supply ofwater treated for silica removal the addition of a rest period to the 5time out for regeneration is as a rule of little.

I p t silica removal units one of which serves to treat Insoluble inhydrochloric acid (s111at the water while the other is regenerated andthen Fe2Os-' 5,1 allowed to stand idle until the silica removal Alzfl 15capacity of the first unit has been exhausted. Ca 3; 45 There are otherways of coagulating the silica MgO i 34.0 or silicates and place theburned mag-nesite with- Ignition loss (COz-|-HaO) 0. in a relativelyshort time in such condition that The following table shows the resultsof a test run under similar conditions as the one described above,except that the water was cold and not The Bio: content of the treatedwater as well as the silica removal capacity of theburn'ed masnesite arequite similar to those obtained in the hot water test, butphenol-phthalein and total alkalinitles are somewhat higher. The lastcol-- of this table shows how the rate of flow was varied during thetest. The results lead'to the conclusion that in the case of cold waterthe rate OrxflOW should not exceed 240 to 400 gallons per 1 hour for a.bed oi 10.6 cubic feet, which-is about it produces a treated water withlow silica content. Upon regeneration and rinsing the magnesite may bebrought in. contact with the solution of a salt which coagulates thesilicates and .thus does away with the colloidal state. For this purposesolutions of the salts of univalent and bivalent bases have been founduseful, e. g., sodium chloride or magnesium sulfate solutions inconcentrations of about 5 to 10 per cent. These are neutral saltsolutions. The best efiect, however, has been obtainedby the use ofsalts which, upon hydrolysis, have an acid reaction and which,consequently, aredecomposed by the magnesite. Examples of such salts arethose of tri-valent bases, especially aluminum and iron. The solutionsof such salts can be used in much more dilute form than those of theneutral salts, concentrations of one and even of one-half percentproducing the desired effect; The quantity of salt required is likewiseappreciably smaller than in the case of neutral salts, 0 .6, pound ofaluminum sulfate, for instance, being sufficient for the treatment ofone cubic foot of burned masnesite. In the use of these saltscoagulation appatently is effected not only by the electrolyte but alsoby the metal hydrates formed upon contact with the magnesite. Whilesalts 01 other tri- One-third to one-flithot the rate used with coedvalent metals, such as chromium salts, may be This can usually abovementioned salts of aluminum and Even burned magnesite which has becomeex- 2 best suited for practical purposes, not hausted in the silicaremoval treatment of hot :ause of their low cost,--but also because ofwater may be restored to a satisfactory condition- :cellent eflect. Eventhe salts of metals for some further use by one of the above inen- Jthervalencies may be employed, for in- 5 tioned coagulation measures, otherthan the apialts of zinc, tin orcopper. In the use plication of heat, orby treatment with water reacting salts the quantityof salt may be at ahigher-temperature than that 01- the hot rger to" such extent that apreceding sepawater beingtreated for silica removal.

atmentwith acid becomes unnecessary As has already been pointed out,several of rbonates, etc., aredecomposedon account the coagulationmeasures may be combined. A

.cid'reaction. "Thei'ormed hydrates a e period of idleness iollowingregeneration is alninated by rinsing.

l -ways beneficial, even when heat or coagulants or furthermore, foundthat'coagulation of I both have been applied in connection with the esmaybe broughtabout by'application regeneration process. 1 The simplestway of doing this is to Prolonged tests in which burnedmagnesite waswater, preferably at boiling temperature. alternately exhausted andregenerated have the bedso that the. burned magnesite shown thatrepeated regeneration of the magnel by'the watch. .In ge neral,aiquantity site increases its silica removal capacitybetween equal to2.5 "to 4 volumes 'ofthe' burned regenerations. The fact may be utilizedfor 'e is suflicient, but'more .water, e. g., 8' initially increasingthe capacity of burned magmay well be used.' The beneficial efnesiteprior to using it for silica removal, by

re hot water is further improved by disbringing it in contact with acid,thus etching or n itelectroiytes such nssthosereferred. activating thegrains. For. a greater'increa'se in preceding paragraph, but in mostcases capacity acid pre-treatment is repeated ,a

if plain hot water is suflicient'for prac. number of times with freshacid or with a, mix-' poses. Another way of applying heat is ture offresh skid used acid.

se of a heating jacket which is placed Another wayof activating theburned magnehe'container forbumed'magnesite and site is to bring it intoprolonged contact, e. g, which hot water or steam is passed. The forone-half to two hours, with hot water, prefr"mi=10yed.'.forthe-coagulation treat- 'erably at. boiling temperature, or steam. The 711881811011 10W 11 11 nt nt that it hot water or steam may be repeatedlycirculated Bed without harm. through the con ainer holding the burnedmags otherwayof eifecting the desired coagnesite undergoing activation,but it is better to i' to treat the burned-'ma'gnesite' with renew thehot water or steam continuously or as in the form ofj'a watery solution.at least repeatedly inorder to bring the mag- 1 generation, rinsing and'subsequent co-' .nesite again in contact with fresh activating mebyheat or the addition of'eoaguiants dium. Good activation is gbtg i nedby combinan be carried out in" a relatively short 1 ing the hot water orsteam gywtlnent with a entire treatment requiring in general treatmentwith acid, e. g.,-

op hioric or acetic ;o one and OHE-hflfhOlll'S. As 'coin- 40 acid,-crCO2 containing water. The two treat-. a'allowingthe magnesite to standidle,. ments may be combined through the use of diid of treatment hasthe advantage that lute hotacid but it is somewlia i. shorterinterruption in the-silica reout the two steps separately, ei i'ormance.Even with these-quick acting. the other. Still better activation isobtained ation measures, however, a short period by alternatelyrepeating the treatments with hot oi,say,'one hour has-been foundbenewater or steam. and with acid two or three or at the entiretreatment, includingv the more'times. i, then requires from two totwoand The duration and intensity of the activating ours. v a treatment tobe used for best results, e. g., temfve mentioned methods of coagulatingperature, acid concentration and number of rep- )1.siiicates;can;.;at;ieast.partlmreplace etitions, depend somewhat uponthe type of raw nerationi" irdinary'v temperatures no longer. pronesite,and the burning t :ated water with sufliciently low silica burned at.very high temperatures apparently reay be'treated for a brief periodwith quiring a stronger activation than one burned at and then willagain produce low silica a lower temperature. rexample, a bed of-burnedmagnesite Y Details of carrying out a good activation treat- .09 cubicfoot wasused to treat cold ment and the improvements in the results ob asilica contentof "-14 parts'per miltamed-thereby are. illustrated by thefollowing i exhaustion ofthis bed to such an example, ,thetr ated,waterc nte ned as uc 0.2 to 0.6 mm. was used which had been. pregi lggis g rzigsiz zpared by burning Austrian magnesite at a tem- -a po-- Iimanvadditiional 130 gallons of perature of about 3180 F The activatingme Thus, w a 'm'agnesite -m ,terial employed in making the burnedmagtemperature, magnesite A burned magnesite of a grain size of y Isilica. free t provided that .diuin was a solution of 37.4. poundshydrochloric 4 acid in 240 gallons oi water. This solution was afterwithdrawing 15 to 20 gallons of Y be! thei magma-site. bed was, allowedheated to about 194. .F. and then repeatedly n wager passed through 10.6cubic ieet of the'magnesite repatem water for a period'of minutes. Thistreatment was finger TS-FOTEQ-th smog removal repeated three more timeswith fresh treating the' burned m gn ia-an i r t solution so that theentire treatment required i ri df 'out; Thu p -1 15 r 1 61-, about twohours. Thereupon the activated maggent ma alt rnate w th n acid nesitehad 2.7 times the silica removal. capacity L treatm nt; th magn a; beingv of the'same, not activated magnesitaand it could :areinoiral'betweensuch-treatments. he used. to treat correspondingly larger quantiassessor ties of water for silica removal between regenerations.

What I claim is:

l. A process of removing silica from water which comprises bringing thewater in contact with burned magnesite, removing the water from contactwith said magnesite, then treating said magnesite with dilute acidreacting solution of at least one of the group of compounds consistingof hydrochloric, sulphuric, carbonic, nitric, acetic and lactic acidsand those salts of iron, aluminum and chromium which hydrolize insolution to give an acid reaction, and then bringing an additionalquantity of water in contact with said magnesite.

2. A process of removing silica from water which comprises reducing thealkalinity of water, then passing said water through a bed of granularburned magnesite to a point of use until the water flowing from said bedcontains appreciable silica, treating said bed with a dilute solution ofan acid from the group consisting of hydrochloric, sulfuric, carbonic,nitric, acetic and lactic acids, rinsing said bed, and again passingsaid water through said bed.

3. A process of removing silica from water which comprises passing waterthrough a bed of granular burned magnesite to a point of use until thewater flowing from said bed contains appreciable silica, then flowingthrough said bed dilute acid reacting solution of at least one of thegroup of compounds consisting of hydrochloric, suli'uric, nitric,carbonic, acetic and lactic acids and those salts of iron, aluminum andchromium which hydrolize in solution to give an acidreaction, until theliquid flowing from the bed contains acid, rinsing the bed free of acid,and again passing water through said bed.

4. A process of removing silica from water which comprises passing waterdownwardly through a bed of granular burned magnesite to a point of useuntil the water flowing from said bed contains appreciable silica, thenpassing up wardly through said bed to waste a dilute acid reactingsolution of at least one of the group of compounds consisting ofhydrochloric, sulfurlc, nitric, carbonic, acetic and lactic acids andthose salts of iron, aluminum and chromium which hydrolize in solutionto give an acid reaction, passing water downwardly through saidbed towaste until the water flowing to waste has a low silica content, andthen again passing water downwardly through said bed to said point ofuse.

5. A process of removing silica from water which comprises passing waterthrough a bed of granular burned magnesite until the water flowing fromsaid bed contains appreciable silica, then flowing through said beddilute acid reactaluminum and chromium which hydrolize in solution togive an acid reaction, then treating said magnesite with HeQat atemperature higher than that of said water, and then bringing anadditional quantity of water in contact with said magnesite;

7. A process of removing silica from cold water which comprises passingcold water through a bed of granular burned magnesite, thereaftertreating said magnesite with a dilute solution of acid from thegroupconsisting of hydrochloric, sulfuric, nitric, carbonic, acetic andlactic acids, thereafter passing hot water through said magnesite, andagain passing cold water through said magnesite.

8. A process of removing silica from cold water which comprises passingcold water through a bed of granular burned magnesite to a point of useuntil the cold water flowing from said bed contains appreciable silica,then passing hot water through said magnesite, then again passing coldwater through said magnesite, then treating said magnesite with a diluteacid reacting solution of at least one of the group of compoundsconsisting oi hydrochloric, sulfuric, nitric, carbonic, acetic andlactic acids and those salts of iron, aluminum and chromium whichhydrollze in solution to give an acid reaction, then treating saidmagnesite with H2O at a temperature higher than that of said water, andagain passing cold water through said magnesite to ing solution of atleast one of the group of oompounds consisting of hydrochloric,sulfuric, nitric, carbonic, acetic and lactic acids and those salts ofiron, aluminum and chromium which hydrolize in solution to give an acidreaction, rinsing said bed free of acid, allowing said bed to stand idlefor a prolonged period without flow therethrough, and again passingwater through said bed.

6. A process of removing silica from water which comprises bringing thewater in contact with burned magnesite, removing the water from contactwith said magnesite, then treating said magnesite with a dilute acidreacting solution of at least one of the group of compounds consistingof hydrochloric, sulfuric, nitric, carbonic, acetic and lactic acids andthose salts of iron,

a point of use.

9. A process of removing silica from water which comprises activatingburned magnesite by a treatment with hot H2O, bringing the water incontact with said activated magnesite, removing the water from contactwith said magnesite, then treating said magnesite with a dilute acidreacting solution of at least one of the group of compounds consistingof hydrochloric, sulfuric, carbonic, nitric, acetic and lactic acids andthose salts of iron, aluminum and chromium which hydrolize in solutionto give an acid reaction, and then bringing. an additional quantity ofwater in contact with said magnesite.

10. A process of removing silica from water which comprises activatingburned magnesite by treating it with a dilute solution of acid from thegroup consisting of hydrochloric, sulfuric, nitric, carbonic, acetic andlactic acids, bringing the water in contact with said activatedmagnesite, nemoving the water from contact with said magnesite, thentreating said magnesite with a dilute acid reacting solution of at leastone of the group of compounds consisting of hydrochloric, sulfuric,carbonic, nitric, acetic and lactic acids and those salts of iron,aluminum and chromium which hydrolize in solution to give an acidreaction, and then bringing an additional quantity of water in contactwith said magnesite.

11. The process of claim 10, the dilute solution of acid being hot.

12. A process of removing silica from water which comprises passing thewater through a bed of granular burned magnesite-to a point of use untilthe water flowing from said bed contains appreciable silica, thereaftertreating said magnesite with dilute acid reacting solution of at leastone of the group of compounds consisting of hydrochloric, sulfuric,nitric, carbonic, acetic and lactic acids and those salts of iron,aluminum and chromium which hydrolize in solution to give an acidreaction, thereafter treating said magnesite with a solution of silicacoagulant from the group consisting of sodium chloride and magnesiumsulfate; and again passing water through said masnesite to a point ofuse.

13. A process of removing silica from water which comprises passing thewater through a bed of granular burned magnesite to apoint of use untilthe water flowing from said bed contains appreciable silica, thereaftertreating-said magnesite with dilute acid reacting solution of at leastone o! the group of compounds consistin; of hydrochloric, sulfuric,nitric, carbonic, aceticand lactic acids and those salts of iron,aluminum and chromium which hydrolize in solution to give an acidreaction, thereafter treating said magnesite with a solution of ammoniato coagulate dissolved silica, and again passing water through saidmagneslte to a point of use. OI'I'O LIEBKNECHT.

